There's a protest planned for this morning outside the courthouse in Sanford, Florida. People say they want justice for the family of Treyvon Martin. Last month, that black teenager was shot by a white neighborhood watch volunteer. The shooter says he acted in self-defense, although the teen he shot was unarmed. And newly released recordings of 911 calls offer painful details of the killing.

We should warn you, this three-minute report from our colleague Joel Rose contains the sounds of those calls, which some may find disturbing.

JOEL ROSE, BYLINE: The first call to 911 comes from George Zimmerman, a neighborhood watch volunteer in Sanford, Florida, just outside Orlando.

(SOUNDBITE OF 911 TAPE)

ROSE: Zimmerman gives the dispatcher a description of a young black man who appears to be, quote, "up to no good," unquote, with his hand in his waistband, looking around at local homes.

(SOUNDBITE OF 911 TAPE)

ROSE: Then Zimmerman tells the dispatcher that the suspicious man is running away.

(SOUNDBITE OF 911 TAPE)

ROSE: But Zimmerman apparently disregarded that instruction. What happened next is in dispute. Another caller to 911 reported two men yelling and fighting outside her house. In the background, you can hear what may be one of the men screaming, followed by a gunshot.

(SOUNDBITE OF 911 TAPE)

(SOUNDBITE OF SCREAMING)

(SOUNDBITE OF 911 TAPE)

(SOUNDBITE OF GUNSHOT)

(SOUNDBITE OF 911 TAPE)

ROSE: What's not in dispute is that Treyvon Martin was unarmed when he was shot in the chest and killed. The 17-year-old from Miami was walking from a convenience store back to his father's house in Sanford. George Zimmerman has not been charged with a crime. He was licensed to carry a concealed gun and told police he was acting in self-defense.

But Martin's family wants Zimmerman arrested for murder. Their lawyer, Benjamin Crump, has been saying that for weeks at press conferences like this one.

(SOUNDBITE OF PRESS CONFERENCE)

ROSE: If the shooter was black and the victim was white, the family says there's little doubt that police would be handling the case differently.

But at another press conference, Sanford Police Chief Bill Lee denied that race is a factor in their decision not to arrest Zimmerman.

BILL LEE: Mr. Zimmerman has made the statement of self-defense. Until we can establish probable cause to dispute that, we don't have the grounds to arrest him.

ROSE: Police say they've handed the case over to the state attorney's office to determine what will happen next. Legal experts say prosecutors may find themselves hemmed in by state law.

JEFFREY BELLIN: Florida law really pushes the envelope of making the right of self-defense as broad as possible.

ROSE: Jeffrey Bellin teaches law at Southern Methodist University. He says Florida is one of 17 states that have what's known as a Stand-Your-Ground Law, meaning you have no obligation to back down in the face of a threat, even when you're outside your home.

BELLIN: As long as you are somewhere you have a lawful right to be, if someone attacks you, the words of the statute are you can meet force with force, including deadly force, if you reasonably believe that that is necessary.

ROSE: That has not discouraged the family of Treyvon Martin. His mother, Sabrina Fulton, spoke at a press conference on Friday.

(SOUNDBITE OF PRESS CONFERENCE)

ROSE: The family is calling on federal authorities to take over the investigation, saying they've lost faith in the Sanford Police Department.